A Photocatalytic Approach for the Synthesis of L-Shape Bicyclic NHC Ligands

L-shape N-Heterocyclic Carbenes (NHCs) based on the imidazo[1,5-a]pyridine (ImPy) scaffold have recently gained considerable interest as the true carbene ligand analogues of the popular dialkylbiarylphosphines, better known as Buchwald phosphines. Nevertheless, the substitution pattern of ImPy ligands is still rather limited due to synthetic access issues. We report herein an efficient and versatile visible light photocatalytic strategy to access L-shape bifunctional ImPy ligands laterally-functionalized by a phenol group. Mechanistic investigations supported by density functional theory (DFT) reveal that the excited state of the iridium photocatalyst undergoes either a reductive quenching (SET process) or an energy-transfer quenching (EnT process) depending on the nature of the counterion of the 5-bromoimidazo[1,5-a]pyridinium substrate salt. Moreover, the bifunctional character of these new family of L-shape ImPy ligands is demonstrated by the preparation of a gold(I) complex exhibiting a free OH function capable of intermolecular hydrogen bonding. This work highlights the advantage of visible light photocatalysis in the synthesis of advanced NHC ligand structures, a strategy that has not yet been considered despite its potential benefits in terms of versatility, diversity and practicability.